1. Field of the Invention
The present invention relates to a fusing device, method and computer readable medium using a heat roller, for use in an image forming apparatus, such as a printer, a facsimile, a photocopier, etc.
2. Discussion of the Background
Generally, a heat roller fusing device included in an electrophotographic image forming apparatus includes a fusing roller with a heater, a pressure roller which contacts and rotates with the fusing roller, and a driving device which rotates the fusing roller. The driving device includes gears and gear driving parts which are provided, for example, at an end portion of the fusing roller.
Toner images formed on a sheet are fused on the sheet under the influence of heat and pressure after passing through a contact part, i.e., a nip part between a fusing roller and a pressure roller. Although the sheet may have wrinkles in a fusing process while passing through the nip part between the fusing roller and the pressure roller, occurrence of wrinkles is generally prevented by making a shape of the fusing roller such that a diameter of both end sides thereof is greater than a diameter of a center part thereof. As a result, the end sides of the sheet are transferred at a faster rate than the center part of the sheet and the sheet is thereby stretched, so that occurrence of wrinkles is typically reduced.
In Japanese Laid-open Patent Publication No. 5-333737, a fusing device is described in which a driving force transmission mechanism is provided to transmit a driving force to a fusing roller when a paper sheet (i.e., single sheet) is transferred between the fusing roller and a pressure roller. When an envelope (i.e., double sheet) is transferred therebetween, the driving force is also transferred to the pressure roller via the driving force transmission mechanism so as to make the transfer speed of the envelope at the fusing roller side and the pressure roller side substantially equal.
In Japanese Laid-open Patent Publication No. 7-261592, another fusing device is described in which, in order to avoid sheet shifting and occurrence of wrinkles even when several thicknesses of sheets and envelops are printed at high speed, a fusing roller and a pressure roller are driven as driving rollers, respectively, when several thicknesses of sheets or an envelop is printed. When, however, a single sheet is transferred, the pressure roller is switched to be the only driven roller.
In the above-described background fusing devices, wrinkles are avoided on the envelop and the several thicknesses of sheets by making the velocity of the surfaces of the fusing and pressure rollers substantially equal.
However, in a full color image forming apparatus, wrinkles may occur on sheets in a two-sided copy mode even though the velocity of the surfaces of the fusing and pressure rollers are kept substantially equal. The toner layers of full color images are typically thicker than those of single color images, and when the toner of the full color images is melted at a nip part between fusing and pressure rollers, a coefficient of friction therebetween significantly drops. In a one-sided copy mode, because there is no toner on a backside of a sheet, the coefficient of friction remains constant and a velocity distribution error is less likely to occur. In a two-sided copy mode, however, because the coefficient of friction at an image portion and a non-image portion is different on each side of a sheet, the velocity distribution error of the sheet transfer speed at a nip part between a fusing roller and a pressure roller occurs, and thereby wrinkles typically are generated on the sheet.